The rare earths have atomic numbers from 57 (La) to 71 (Lu), and comprise the elements across which the 4f orbitals are filled: that is, lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). They have atomic configurations [Xe]6s25d14fn or [Xe]6s24fn+1, with n varying from 0 for La to 14 for Lu. Their most common ionic charge state is 3+, with the 4f levels spanning the Fermi energy. They are the only stable elements with more than marginally filled f-shell electronic orbitals and, as a consequence, they are the elements with the largest spin and orbital moments. In ordered solids they contribute to the most strongly ferromagnetic materials, a contribution that has ensured their utility in technologies that require strong permanent magnets. Despite their name they are by no means rare, with the exception of promethium, which has no stable nuclear isotope.
The rare earth nitrides form in the face-centered cubic NaCl structure with lattice constants ranging from ˜5.3 Å for LaN to ˜4.76 Å for LuN, in total a 10% difference across the series and about 0.7% between nitrides of neighbouring atomic species. The rare earth nitrides were first investigated in the 1960s, when technological developments overcame the problems faced in separating the chemically similar members of the rare earth series. The rare earth nitrides have interesting magnetic and electronic properties. The rare earth nitrides have an optical bandgap typically of the order of 1 eV and are almost all ferromagnetic, with magnetic states that vary strongly across the series and coercive fields depending strongly on the growth conditions. For example, SmN is the only known near-zero-moment ferromagnetic semiconductor, with an enormous coercive field, and GdN has a coercive field some three orders of magnitude smaller.
The rare earth nitrides show promise in applications as diverse as spintronics, infrared (IR) detectors, and as contacts to group III nitride semiconductor compounds. For example, rare earth nitrides have been used in the fabrication of spin-filter Josephson junctions and field effect transistor structures.
The rare earth nitrides are also epitaxy-compatible materials with group III nitride semiconductors, a technologically important family of materials for the fabrication of, for example, optoelectronic devices and high power transistors. The properties of the rare earth nitrides are also complementary with those of the group III nitrides. A heterojunction involving these two semiconductor materials could have very attractive properties for multi-wavelength photonic devices and spin light emitting diodes. For example, GdN quantum dots have been shown to enhance the efficiency of GaN tunnel junctions.
Semi-insulating and insulating rare earth nitride layers, in particular, could be useful, optionally in combination with group III nitrides, in the fabrication of, for example, spintronics, electronic and optoelectronic devices. Such layers may avoid, for example, leakage current or degradation of radio frequency performance of such devices.
High quality epitaxial thin films of rare earth nitrides can be grown using ultra-high vacuum (UHV)-based methods, such as molecular beam epitaxy (MBE), pulsed-laser deposition (PLD), and DC/RF magnetron sputtering. However, such UHV-based methods typically result in unintentionally doped films that have a resistivity of the order of 0.05 to 10 mΩ·cm at room temperature and an n-type residual doping concentration associated with a background electron carrier concentration ranging from 1020 to 1022 cm−3, which originates from nitrogen vacancy and depends on the growth conditions.
Accordingly, it is an object of the present invention to go some way to avoiding the above disadvantages; and/or to at least provide the public with a useful choice.
Other objects of the invention may become apparent from the following description which is given by way of example only.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date.